Structure of Image Sensor
As the applications of the optoelectonic device become more and more popular, the demands for the image sensing device increase rapidly. In general, typical image sensors can be categorized into two main parts, which are the charge coupled device (CCD) and the complementary metal-oxide semiconductor (CMOS).
An image sensor is used for recording a change of a photo signal formed by an image and converting the photo signal into an electronic signal. After recording and processing the electronic signal, a digital image is generated for further outputting or recording. In general, the image sensor is formed by a plurality of photo sensing devices, which are either CCD elements or COMS elements.
A CCD image sensor is formed by a capacitor array having a plurality of metal oxide semiconductor (MOS) elements arranged densely. The manufacturing of the CCD is to deposit a silicon oxide layer on a N type (or P type) single crystal silicon substrate. Then, a PN type MOS capacitor receiving a photo signal is formed on the silicon oxide layer. The MOS capacitor is used for converting the photo signal into an electronic signal. Moreover, the dielectric layer and the signal transmitting circuit are arranged in the boundary of the MOS capacitor array and then integrated into the CCD elements on the single crystal substrate with the powering device. Thus, a CCD image sensor is accomplished.
On the other hand, the CMOS imager sensor is a semiconductor for recording the change of the photo signals. The CMOS mainly includes silicon (Si) and germanium (Ge), so that the N type and P type semiconductors can exist in the CMOS element. Therefore, the currents can be generated by these two complementary semiconductors. After processing and recording such currents, a digital image can be outputted or recorded. The difference between the CCD and CMOS is that the CCD element is formed on the single crystal semiconductor substrate while the CMOS element is formed on the metal oxide semiconductor substrate. However, the working principle of CCD and COMS are identical.
In addition to the abovementioned semiconductor photo sensing device included in the image sensor for converting the photo signals into electronic signals, a plurality of color filters are also included in the image sensor in order to output the color image. Typically, the color filter array included in the image sensor can be either the RGB color filter array or YMC color filter array.
In the conventional technology, the color filter array is disposed above the semiconductor photo sensing device. Furthermore, a micro lens array, such as a convex array, is disposed above the color filter array for converging or condensing the incident light. With the aid of the convex array, the incident light can be condensed to a smaller beam and projected to the specific area of the semiconductor photo sensing device, so as to increase the photosensitivity of the image sensor. Therefore, the typical arrangement of the image sensor, which is formed either by CCD or CMOS semiconductor photo sensing device, mainly includes a lens array, a color filter array and a semiconductor photo sensing device array arranged in sequence along a direction of the incident light.
Please refer to FIG. 1, which shows an arrangement of the conventional CMOS photo sensing element. As can be seen from FIG. 1, the CMOS photo sensing element 10 includes a substrate 11, a first photodiode 12a, a second photodiode 12b, a third photodiode 12c, a metal opaque layer 13, a first color filter layer 14a, a second color filter layer 14b, a third color filter layer 14c, a micro lens layer 15 and a light beam 16. Typically, the first, second and third color filter layers are used for filtering the green, red and blue light respectively.
No matter it is a CCD or a COMS image sensor, a plurality of photo sensing elements, which is also called pixels, are included therein for constructing a hundred thousand level or a million level image sensor. For a CCD image sensor, the electronic photo signals generated in every pixel of each column is transmitted to a buffer in sequence, and then outputted to an AC/DC (ADC) converter disposed near CCD photo sensing elements for amplifying and digitizing the analog electronic signals. The amplified and digitized signals are then transmitted to a processing chip. However, for a COMS image sensor, each pixels is collocated with an ADC converter, so as to amplify and digitize the electronic signal generated by each COMS pixel directly. Therefore, the main differences between the CMOS image sensor and the CCD image sensor are the disposition and the number of the ADC converter.
Please refer to FIG. 2, which shows the pixels layout of a CMOS image sensor, which includes a CMOS photo sensing element and an ADC converter in each pixel. As can be seen from FIG. 2, the CMOS image sensor 20 includes a plurality of pixels 21, each of which has a CMOS image sensor 22 and an ADC converter 23. The feature of the COMS image sensor is that each CMOS image sensor 22 is collocated with an ADC converter 22, so that the electronic signal generated by each COMS pixel can be amplified and digitized directly, and then transmitted to a processing chip for digital signals processing.
Optical Crosstalk, Brightness Difference and Pixel Layout Uniformity
No matter it is the CCD or CMOS image sensor being used, the optical crosstalk effect is always a problem to the designer of the image sensor. The optical crosstalk effect means the incident light transmitted into a pixel is deflected to the adjacent pixel(s), so that the additional photo energy is absorbed by the adjacent pixel(s) and thus the original photo energy which should be sensed by the adjacent pixel(s) will be affected by the deflected incident light.
Please refer to FIG. 3, which schematically explains the crosstalk effect of an image sensor. As can be seen from the FIG. 3, the conventional image sensor 30 includes a first, a second and a third micro lenses 31a-c, a first, a second and a third color filters 32a-c, a light shield 33, an IC stacking layer 34 and a first, a second and a third photodiodes 35a-c. 
In a normal condition, a normal incident light 37a passing through the second micro lens 31b should be projected to and absorbed by the second photodiode 35b. However, in a crosstalk condition, a crosstalk incident light 37b having a larger incident angle 38b passing through the second micro lens 31b will be projected to and absorbed by the first photodiode 35a. Therefore, the crosstalk effect is dependent on the respective incident angles 38a, 38b of the incident lights 37a, 37b. 
The brightness difference also results from the difference of the incident angle. The generated electrical signal in each photodiode is related to the sensed intensity of the incident light. However, the sensed intensity of the incident light varies with the incident angle of the incident light. Therefore, the incident angle also effects the brightness difference in each photodiode.
Please refer to FIGS. 12 and 13 which show the top views of two pixel layouts of the COMS image sensor, respectively. As can be seen from FIG. 12, an uniform decenter pixel array 110 includes a plurality of pixels 111, each of which includes a CMOS photodiode 112 and an ADC converter 113. The uniform decenter pixel array 110 further includes a micro lens array, so that each micro lens 114 is formed on each of the pixels 111 for converging the incident light into the CMOS photodiode 112. In such an uniform decenter pixel array 110, the pixel layout is manufactured in a 0.35 μm process, and the area ratio of the CMOS photodiode 112 to the ADC converter 113 in each pixel 111 is about 0.4˜0.6. Since the photodiode 112 in each pixel 111 is disposed in the lower portion of the pixel, each micro lens 114 should be disposed decenteredly, so as to converge the incident light into the CMOS photodiode 112.
On the other hand, when the process of the CMOS pixel array is updated from the 0.35 μm process to the 0.13 μm one, an non-uniform decenter pixel array 120 is provided, as shown in FIG. 13. The non-uniform decenter pixel array 120 includes a plurality of pixels 121, each of which includes a CMOS photodiode 122 and an ADC converter 123. The non-uniform decenter pixel array 120 further includes a micro lens array, so that each of micro lens 124 is also formed on the each pixel 121 for converging the incident light into the photodiode 122. In such an non-uniform decenter pixel layout processed in 0.13 μm, four pixels are grouped into a pixel set, in which the four photodiodes 122 are disposed adjacent to one another and the ADC converters 123 are disposed around the four photodiodes 122 in each pixel set. However, the area ratio of the CMOS photodiode 122 to the ADC converter 123 in each pixel 121 is also maintained at about 0.4˜0.6. However, since four photodiodes 122 in each pixel set are grouped together, the layout of the micro lens array 124 is changed to an non-uniform decenter layout, which may result in the overlapped layout of the micro lens array 124, as shown in FIG. 13.
Since the optical crosstalk effect, the brightness difference effect and the pixel layout uniformity are the factors affecting the image quality of the image sensor, most manufacturers are devoted themselves to find the solutions for improving the image quality of the image sensor. In the prior art, there are two known technical schemes which can be used for improving the image quality of the image sensor. The two technical schemes are described as follows.
Reference 1: Taiwan Patent No. TW200525773
An image sensor which can receive an uniform photo energy in a chip, especially in the areas between the central and the boundary of the chip, is provided in the reference 1. A further aspect of this image sensor is to provide an image sensor for solving the problem resulting from the photo crosstalk effect.
The image sensor provided in the reference 1 includes a micro lens layer having a plurality of micro lenses, each of which is corresponded to a sensing area of a sensor chip. The feature of such an image sensor is characterized in that the size of each micro lens is increased with the distance between the lens and the central of the sensor chip, so that the photo sensing uniformity in each sensing area of the sensor chip can be achieved.
Furthermore, please refer to FIG. 4, which shows a further image sensor layout provided in the reference 1 for abating the problem resulting from the photo crosstalk effect. As can been seen from FIG. 4, the lower part of the drawings is the conventional design of the image sensor, while the upper part of the drawings is the improved design of the image sensor. In the conventional design, each micro lens 42 is aligned with its corresponding color filter 43, IC stacking layer 44 and sensing area 45, respectively. However, in the improved design, the dispositions of the lenses are varied with their distance from the center of the sensor chip center. Since it is very complicated and inefficient to dispose every micro lenses, the micro lens layer 42 are categorized into several groups according to their distances from the center of the sensor chip center. As can been seen from FIG. 4, the micro lenses in the Group 1, which is adjacent to the center of the sensor chip, keep in the same disposition as those in the conventional design. As to Groups 2 and 3, which are further away from the center of the sensor chip, the micro lens set in each group are shifted (the shift distances in this case are 0.07 μm and 0.14 μm for Group 2 and Group 3, respectively) in a direction toward the center of the sensor chip, so that the effect results from different incident angles can be mitigated.
Reference 2: U.S. Pat. No. 6,803,250
In the reference 2, an image sensor with a complementary concave and convex lens layers is provided. Please refer to FIG. 5, which shows an image sensor structure provided in the reference 2. As can be seen from FIG. 5, the image sensor 50 includes a substrate 51 with a photoactive region 52 embedded therein. A first planarizing passivation layer 53 having a pair of patterned first conductor layers 54a, 54b formed therein and a second planarizing passivation layer 55 having a pair of patterned second conductor layers 56a, 56b formed therein are disposed above the substrate 51 in sequence. A color filter 58 is further disposed above the second planarizing passivation layer 55 with a first spacer layer 57 formed therebetween. A convex lens 510 is further disposed above the color filter 58 with a second spacer layer 59 formed therebetween. The first and second planarizing passivation layers 53, 55 are formed of a dielectric material transparent for adjusting the refraction angle of the incident light. With such a configuration described above, the first and second planarizing passivation layers 53, 55 are operated as a concave lens to be in combination with the convex lens 510 for enhancing the optical performance of the image sensor. However, such an image sensor still has the problem relating to the photo crosstalk effect and the brightness difference.
Base on the above, the conventional image sensors still have the problems in eliminating the crosstalk effect and the brightness difference. Therefore, it is necessary to provide a novel technical scheme to solve the abovementioned problems.